The invention relates to a process for ultrasonic testing particularly of a specimen in the form of a railway wheel in accordance with the features set forth in the preamble of claim 1.
Such a process for ultrasonic testing is known from U.S. Pat. No. 3,978,712 according to which two ultrasonic probe elements are used. The two probe elements are arranged opposite one another and angled to a normal to the rail in the contact zone of a railway wheel. The one probe element is used to emit the ultrasonic waves at a predefined angle to the normal and introduce them into the railway wheel in circumferential direction, with the ultrasonic waves traveling through the railway wheel in circumferential direction. The ultrasonic waves, which propagate linearly in principle, are intended to be deflected into circular wave propagation due to reflection along the wheel perimeter or the wheel-air boundary layer. After at least one rotation, a portion of the ultrasonic waves exits from the wheel in the contact zone and reaches the second probe acting as a receiver. This forced circular wave propagation causes substantial losses so that the first probe element embodied as the transmitter must be correspondingly large and designed for high acoustic power. Furthermore, due to the reflections along the wheel perimeter required for circular wave propagation, a high interference level must be expected so that the evaluation of the ultrasonic waves received by the second probe element and detection of useful signals corresponding to flaws in the wheel requires considerable complexity. Ultrasonic waves striking flaws within the railway wheel and suitable for subsequent evaluation are not reflected at the flaws but are attenuated and transmitted in circumferential direction until they reach the second receiver probe element.
Furthermore, DE-A 195 44 217 discloses a process for ultrasonic testing of a specimen, particularly in the form of a railway rail over which a test wheel rolls. The test wheel, in the area of its substantially cylindrical rolling surface, which rolls over the specimen, contains a plurality of probe elements, a number or subgroup of which form a probe. The transmit or receive sides facing the probe come to lie within the contact zone of the rolling surface and the surface of the specimen. The probe elements of the respectively activated probe are operated in what is referred to as a phased array technique making it possible to predetermine an angled introduction of the ultrasonic waves into the test specimen through time-defined, runtime delayed control. A detection unit comprising particularly a laser and a photo detector detects the angular position of the test wheel. Said detection unit does not detect either the speed or the reaching of any given zone. The test wheel further comprises a number of distance sensors to isolate the probe elements forming the respectively active probe if a predefined minimum distance to the travel surface fails to be reached. The arrangement of said distance sensors and their signal processing for isolation require considerable additional complexity. A cylindrical test specimen cannot readily be inspected and the power supply, which powers the test wheel rolling over the rail, either via sliding contacts or an alternating voltage supply, requires additional measures.
Based thereon it is the object of the invention further to develop the process for ultrasonic testing of the aforementioned type to ensure functionally reliable detection of flaws within the volume and on the surface of the specimen during the rollover of a specimen, particularly a railway wheel. The complexity required to handle and evaluate the test should be reduced to a minimum. Reliable testing of railway wheels made of different parts and/or different materials should be possible and the detection particularly of internal cracks starting at the wheel disk / wheel rim junction should be highly reliable.
This object is attained in accordance with the invention as described and claimed hereinafter.
The process for ultrasonic testing according to the invention is distinguished by its reliable detection of flaws, particularly internal defects, such as cracks in the test specimen. For instance, if the specimen is embodied as a railway wheel consisting of different parts and/or different materials, defects starting, for example, from the junction zone between wheel disk and wheel rim can be detected. In a recess or groove of the railhead, a linear array of an arrangement of ultrasonic probes is provided, which inject ultrasonic waves into the specimen preferably without coupling means. The angle of incidence of the ultrasonic waves can advantageously be predefined. The length of the array is at least equal to the circumference of the test specimen, and the probe elements preferably take the form of electromagnetic ultrasonic transducers. Testing takes place during the rollover of the specimen. A detection unit comprising particularly two position detectors, such as photoelectric barriers, detects the rolling in of the specimen into the probe array path and determines the speed of the specimen or wheel. Advantageously, the determined and/or calculated speed is used to control activation or deactivation synchronous with the wheel motion of a predefined number of elements of the array, which form a subgroup. The number M of the corresponding elements of a subgroup is predetermined as a function of the largest permissible distance between the two peripheral elements and the wheel surface and/or the required directional characteristics and/or the required test sensitivity. Preferably, the number M is predefined between 3 and 11, particularly between 5 and 9.
As the specimen or wheel rolls into the probe array path, the first subgroup of the linear array with M individual elements is activated in such a way that the contact point between wheel and rail is symmetrically in the center of the subgroup. The activation/trigger instant (master trigger) is calculated from the previously determined speed of the wheel and the distance from the center of the subgroup to the position of the photoelectric barriers. This function is performed by a trigger module. Through an electronic runtime delay in the excitation of the individual subgroup elements, the ultrasound is coupled into the wheel as a transverse wave with horizontal polarization at predefinable angles of between 35xc2x0 and 90xc2x0 to the surface normal. The corresponding trigger instants of the array elements are calculated from the master trigger. This electronic swing of the sound beam produced by the subgroup makes it possible to detect defects within the volume as well as on the running surface and along the inside of the wheel. As the wheel moves over the rail, a respective element of the subgroup is switched off and a new element is added as calculated from the master trigger, the determined wheel speed and the known distance of the array elements. Due to this procedure and a multiplexer, which is controlled via the trigger module, a channel number M is sufficient for the ultrasonic test device. This considerably simplifies the circuit complexity. Due to the rolling of the specimen over the rail containing the probe elements, the relative speed at the contact point, i.e. for the center element of the subgroup, is exactly zero and for the subgroup elements to the right and left thereof is very low. Consequently, there is no acoustic Barkhausen noise interference due to magnetic reversal as a result of spatially in homogeneous magnetic fields of the probe element.
In a preferred embodiment, the individual elements of the linear probe array are accommodated in a recess or groove in the rail to protect them from mechanical wear. The position of the specimen, and thus the position of the probe elements in relation to the specimen, can be determined by said trigger module whose output signals are stored in a control and evaluation circuit together with the measuring signals received by the probe elements, so that defects within the specimen can be pinpointed based on a reference position after completion of the test.
Further developments and special embodiments of the invention are set forth in the dependent claims.